MachineState must enable MachineAgent monitoring

[Gustavo Niemeyer]

The MachineState should enable the MachineAgent reponsible for this machine to be monitored.

Here is a suggested API:

class MachineState:

    def has_agent(self):
        """Return boolean deferred informing whether an agent is connected."""

    def watch_agent(self):
        """Return two boolean deferreds informing whether an agent is connected, and whether a change happened."""

    def connect_agent(self):
        """Inform Ensemble that this is a machine agent alive."""

This is needed by the MachineAgent (bug #617042) and future work in the ProvisioningAgent (bug #616565).

[Kapil Thangavelu (hazmat)]

i'm working on this in conjunction with generic agent monitoring for domain objects, likely as a mixin state class.